You follow the same procedure as you did part a. At point B (right before it slips) you know the total acceleration (magnitude and direction.) What forces are acting on the object? What information does being on the verge of slipping tell you? From your free body diagram you can write the components of the forces; some of these will contain theta and you can then solve for theta. What do you get?

Hi, Jason. It's been a while but I will try to get you started. I will assume that you have already drawn your FBD and KD for when the block is just about to slip. Breaking Equilibrium equations into radial and transverse components should be your next move. What do have so far?